Agile security using an incremental security architecture

The effective provision of security in an agile development requires a new approach: traditional security practices are bound to equally traditional development methods. However, there are concerns that security is difficult to build incrementally, and can prove prohibitively expensive to refactor. This paper describes how to grow security, organically, within an agile project, by using an incremental security architecture which evolves with the code. The architecture provides an essential bridge between system-wide security properties and implementation mechanisms, a focus for understanding security in the project, and a trigger for security refactoring. The paper also describes criteria that allow implementers to recognize when refactoring is needed, and a concrete example that contrasts incremental and 'top-down' architectures.